As is known, stereolithography is extensively used for rapidly prototyping three-dimensional, even complex objects, as it allows said objects to be manufactured in a very short time and practically with no need to use any special equipment.
In general, the stereolithography technique includes a first virtual division of the geometry of the object to be reproduced in layers having a predefined thickness, which are then actually produced by a stereolithography machine that places them one on top of the other in order to create the object.
The layers are made of a liquid resin that can solidify permanently under the effect of suitable stimulation.
More precisely, the resin is spread on a supporting surface in the thickness corresponding to one layer of the object to be reproduced and then is selectively exposed to stimulation in the areas corresponding to the volume of the object.
A plastic-based resin is generally used and its solidification is obtained by polymerization through exposure to a laser beam.
According to a first known construction form, a stereolithography machine comprises a vat containing the liquid resin, in which a vertically moving platform that supports the object being produced is immersed.
The platform is lowered until covering each layer of the object with a layer of liquid whose thickness corresponds to that required for the successive layer. The liquid layer is then subjected to stimulation in the areas corresponding to the volume of the object to be manufactured, so that it solidifies and adheres to the underlying layer.
The process for manufacturing the three-dimensional object proceeds analogously for the successive layers, with the platform being progressively lowered.
The known technique described above poses the drawback that it is rather difficult to control the thickness of the layers.
In fact, the reduced thickness of each layer, which is generally just a few fractions of a millimeter, and the viscosity of the base material in the liquid state hinder the distribution of the liquid resin on the object being produced.
Consequently, the thickness of the layers often lacks uniformity and this leads to the production of a finished object whose dimensions are imprecise.
Various devices are known, which tend to limit the above mentioned drawback, substantially based on the use of spatulas that level the surface of the liquid resin, but none of them solves the problem in a satisfactory way.
A further drawback posed by the known technique described above is represented by the need to prepare a certain number of supports for the object being produced and arrange them on the platform.
The supports are necessary both to bear the dead load of the object, especially in the case where it has parts that projects towards the outside, and to support the parts that, in particular configurations of the object, are initially separated from the body of the object itself.
By way of example, the dark area of FIG. 1 schematically illustrates a generic three-dimensional object produced according to the known art, which is indicated by W′, while the hatched areas show the above mentioned supports S related to it.
The supports also make it possible to oppose the stress which is generated during the polymerization of the layers and which, due to the reduced thickness of the layers themselves, would deform them in an unacceptable manner.
The supports, however, pose the drawback that they require an additional design stage, which increases the objects overall production time and costs.
The supports also pose another drawback, lying in that they cannot be reused for the production of other objects and therefore are wasted, thus further increasing the cost of the object.
Furthermore, when the object to be manufactured is particularly complex, it is not possible to produce the supports, which actually limits the applicability of the stereolithography technique.
According to a known technique, the supports are produced by the stereolithography machine at the same time as the three-dimensional object, thus making up an integral part of the latter.
Obviously, in addition to the drawbacks already described, the above mentioned technique poses a further drawback, represented by the fact that it is necessary to remove the supports mechanically, with a consequent further increase in the cost of the object.
Furthermore, the removal of the supports involves the risk of breaking the object, which is another drawback.
In addition to the above, there is a further drawback represented by the slowing down in the manufacture of the object, as the stimulation of the base material must be carried out also in the areas corresponding to the supports.
In the attempt to overcome the above mentioned drawbacks, another technique has been developed, which uses a resin that at room temperature is in the form of jelly and therefore is substantially stable from a dimensional point of view.
According to this construction technique, the jelly resin is heated in order to liquefy it, so that it can be spread on the object being produced.
Therefore, differently from the previous case, the object is not immersed in a liquid resin.
Once spread on the object, the resin cools down and forms a jelly layer that is relatively stable at room temperature and is then polymerized by means of a selective stimulation procedure analogous to that already described.
The process is repeated for the successive layers, until completing the object, which is finally heated in order to obtain the liquefaction of the non-polymerized resin and thus extract the finished object.
Advantageously, the technique described above makes it possible to avoid the use of supports for the object being produced, which in fact is supported by the surrounding jelly resin.
However, the above mentioned technique poses a series of drawbacks, among which is the fact that it is rather slow, due to the time necessary for each deposited layer to cool down.
A further drawback lies in that the resin undergoes partial deterioration due to the double heating cycle to which it is subjected, during the spreading of each layer and at the end of the production process, to the detriment of the mechanical characteristics of the object.
The above mentioned drawback, which means that the excess resin cannot be used again to produce other objects, together with the already mentioned slowness of this technique, further increase the cost of the finished object.
Furthermore, analogously to the first known technique previously described, this second known technique does not resolve the problem regarding the control of the thickness of the layers, either, but can even worsen it.
In fact, during the spreading of each layer, the viscosity of the resin is very high, which can generate irregularities on the surface of the layer itself.
The present invention intends to overcome all the drawbacks of the known art outlined above.